The present invention relates in general to nickel-cadmium batteries and more particularly to an improved method for making nickel electrodes for use in such batteries wherein porous plaques are impregnated with active material in a very short time by a cathodization process and wherein the impregnating electrolytic solution is not contaminated and does not generate solid particulate by-products.
The most expeditious method of fabricating battery electrodes for nickel-cadmium batteries is to use a porous nickel sponge-like material and then impregnate them with a suitable active material. For the positive (nickel) electrode, the active material may be nickel hydroxide, Ni(OH).sub.2, and for the negative (cadmium) electrode, cadmium hydroxide, Cd(OH).sub.2, is customarily employed. Both such materials are solids and cannot be introduced into the minute pores of the porous plaques directly as they readily decompose before melting, when heated, and they are not soluable in the sense that they cannot be dissolved and then recovered by evaporating the solvent. Accordingly, the active materials must be deposited indirectly in some way. Usually, this means by some impregnation process.
One of the early and more commercially successful impregnation processes requires the introduction of the active material in the plaque pores by precipitating the appropriate hydroxide from a concentrated solution of the metal nitrate under vacuum. After a suitable soaking period, the vacuum can then be broken and the excess solution drained off. The plaques can then be transferred to electrolytic cells containing a solution of an alkali metal hydroxide at some elevated temperature, say, in the range of 70.degree. to 100.degree. C. This precipitates the corresponding hydroxide in the pores. Further, the plaques are usually made cathodic for a given period of time between nickel sheet anodes to ensure that all of the nitrate present is converted to hydroxide. The plaques may then be washed free of caustic and any loosely adherent deposits of active material brushed free or cleared off with jets of deionized water. A disadvantage of this method, however, is that the process must be repeated over and over again until the desired loading is achieved, normally up to 4-5 cycles or more.
One way of improving on the foregoing process is to employ cathodic precipitation. The actual impregnation of the nickel plaque sheets can be effected in minutes as compared to hours or even days in the more conventional method above described. The cathodic process involves a pH change at the surface of the sintered plaque serving as a cathode which is sufficient to precipitate the metal hydroxide from the electrolyte solution used, conventionally a metal nitrate solution, such as nickel or cadmium nitrate. Solution temperature, concentration and current density are all variables which must be carefully controlled if optimum results are to be obtained.
Additionally, and of particular importance, is the pH. Since the metal anion of the impregnating solution is reduced electrochemically, a zone of high pH is created at the plaque-solution interface. If counterfactors are not introduced, the bulk pH of the electrolytic solution undergoes rather abrupt and substantial changes. The result is a substantial slowing of the precipitation of the metal hydroxide serving as the active material in the plaque structure.
A particular solution to this problem is the inclusion of a buffering or support agent in the electrolytic solution. One known process utilizes an alkali nitrite, such as sodium or potassium nitrite, in sufficient amount to prevent oxygen evolution at the anode and thereby stabilizing the bulk pH of the solution. It nevertheless gives rise to a solid by-product, namely, sodium or potassium nitrate, which continues to build up in the impregnation solution to a point where the solution must either be purified or else discarded.